1. Field of the Invention
The invention relates generally to image transfer between devices. More specifically, a non-destructive method is taught for expanding or contracting images transferred from a first to a second device having incompatible resolution formats.
2. Background of the Invention
Exchange of image data between equipment, input from devices such as scanners, facsimile machines and computers, to output devices such as printers, video monitors, plotting devices or mass storage systems such as floppy discs, fixed discs, CD ROM and the like, is complicated by the inconsistent image proportions of the associated equipment. Expansion or contraction of the image for outputting to a device requires extensive reworking of the image dot/pixel by dot/pixel or data block by data block, in the case of compressed image data. Most known image conversion processes are destructive to the original image in that they randomly discard picture elements (pixels) to obtain the necessary conversion ratio. They generally utilize a multiplication factor, obtained by the ratio of the input to the desired output dimensions, and apply the factor randomly to achieve the desired dimensions. Systems which selectively discard pixels can be equally destructive to the images, though critical information is not usually lost. These commonly used practices result in output of degraded and distorted images. In addition, maintenance of the input aspect ratio, when desired, is virtually impossible utilizing the known image conversion technology. Moreover, the accuracy of past multiplication systems is suspect given the inability of systems to account for fractional remainders of picture elements in a whole integer pixel mosaic, particularly in instances of image reduction.
U.S. Pat. No. 4,628,534 of Marshall, entitled "Method for Changing the Resolution of Compressed Image Data" provides one so-called multiplication method of image conversion. In that method, the compressed image data is multiplied by the ratio of the desired output image size to the original input image size. Fractional "residues" of the multiplication step are rounded up to whole integers if greater than 0.5, or carried as cumulative residue if less than 0.5. Thus, Marshall does attempt to provide additional accuracy by accounting for the fractional residue of the multiplication procedure. The rounding of fractional, or decimal, residue does provide better image reproduction but allows up to 50% error per pixel, which in some applications may be unacceptably high.
It is therefore an objective of the present invention to provide an accurate non-destructive method of image size conversion.
It is a further objective of the invention to provide conversion of an image of one size to another size without distortion or loss of information.
It is yet another objective of the invention to provide an image size conversion method which can be used either to maintain aspect ratio or to convert the dimensions of an image in a non-uniform ratio to the original dimensions.
An additional objective is to provide a more accurate image conversion method which accounts for fractional remainders of picture elements to a higher degree of resolution than previously found in the art.